The present invention generally relates to photolithographic patterning of semiconductor devices and more particularly to a reticle used for the photo lithographic patterning process to form a device pattern on a semiconductor wafer by an ultraviolet or deep ultraviolet light.
The large scale integrated circuits (LSIs) has increased the integration density by about four times in the last three or four years. The demand for increasing the integration density persists and various patterning processes are under investigation for forming extremely fine semiconductor patterns on a semiconductor wafer with an improved resolution.
The photolithographic patterning process that uses ultraviolet or deep ultraviolet lights for the patterning is one of the major patterning processes currently used. This process uses a mask or reticle carrying thereon a semiconductor pattern in an enlarged scale and is particularly advantageous for mass producing the semiconductor devices with large throughput. On the other hand, the process relies upon the photochemical reaction in the photoresist caused by the ultraviolet light that has passed through the reticle and associated therewith, has an inherent problem of poor resolution due to the diffraction of light at the edge of the reticle pattern. This problem becomes particularly acute when the size of the semiconductor pattern to be written on the wafer is decreased in correspondence to the increased integration density. Although various efforts are made to minimize the problem of unwanted diffraction such as use of the deep ultraviolet lights having shorter wavelengths or increase of the numeric aperture of the exposure system, such improvements are approaching the limit.
The reticles for the photolithographic patterning have conventionally been produced by patterning a metal layer grown on a substrate by electron beam or ultraviolet beam. The reticle thus produced is used in a photolithographic exposure system for selectively interrupting an ultraviolet or deep ultraviolet beam used for exposing the photoresist covering the wafer, according to the desired semiconductor pattern.
In order to suppress the Fresnel diffraction of the light that has passed through the reticle, there is a proposal to provide a phase shift region on the pattern on the reticle at selected locations to cancel out the diffracted light beam (Levenson, M.D. et al., "Improving Resolution in Photolithography with a PhaseShifting Mask," IEEE Transactions on Electron Devices, vol.ED-29, no.12, December, 1982.)
FIGS.1(A)-1(C) show the principle of suppressing the Fresnel diffraction proposed in the foregoing reference.
Referring to FIG.1(A) showing an exposure process using a conventional reticle, a glass substrate 1 is provided with an opaque pattern 3 of chromium and the like that selectively interrupts an ultraviolet light designated as UV according to a desired semiconductor pattern. Further a phase shift pattern 2 is provided between a pair of adjacent opaque patterns 3 for shifting the phase of the ultraviolet light passing therethrough with respect to the ultraviolet light that has passed through the reticle without being modified the phase by the phase shift pattern 2.
FIG.1(B) shows a typical distribution of the electric field E of transmitted light beam that has passed through the reticle and FIG.1(C) shows the corresponding distribution of the intensity of the light beam, wherein the solid line represents the case where the phase shift pattern 2 is provided and the broken line represents the case where no such phase shift pattern is provided. As can be seen from FIG.1(B) and FIG.1(C), the transmitted light beam has a finite intensity even in the region immediately under the opaque pattern 3 when the phase shift pattern 2 is not provided, because of the diffraction of the light. On the other hand, when the pattern 2 is provided, the phase of the light beam that has passed through the pattern 2 can be made opposed with respect to the light beam that has passed through the reticle without being shifted the phase thereof. Thereby, a zero-crossing appears in the distribution of the electric field in correspondence to the opaque pattern 3 as shown in FIG.1(B) by the continuous line, and a sharply defined, high resolution pattern shown in FIG.1(C) by the continuous line, is projected on the wafer.
Conventionally, the phase shift pattern 2 has been formed of an insulating material such as silicon oxide or organic material. In forming such a pattern by an electron beam lithography, there has been a problem in that electrons are accumulated in the insulating film forming the pattern 2 as well as in organic resist 4 as shown in FIG.2, and the electron beam is offset from the aimed point because of the coulomb repulsion. Because of this problem, the conventional reticle could not be formed with satisfactorily high resolution.